usb: dwc3: gadget: properly account queued requests

Some requests could be accounted for multiple
times. Let's fix that so each and every requests is
accounted for only once.

Cc: <stable@vger.kernel.org> # v4.8
Fixes: 55a0237f ("usb: dwc3: gadget: use allocated/queued reqs for LST bit")
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>